If they're going twice your speed, they can go twice as far. Assuming they're personally providing at least half the power over the course of the bike ride, they're expending identical calories to you, possibly more to compensate for their heavier bike.

It's true that the moment-to-moment intensity is lower, but lowering intensity is some of the most common advice given to people struggling to get into a sport without their body rebelling, and the challenge as often as not is that they're bored at the beginner's pace and want to push more.

It's the same concept as assisted pull ups or knee pushups. Sure, you can build up from scratch by just doing a fraction of the duration, but that's boring and frustrating. Being able to actually move your body the full distance with less effort is a much more engaging way to build up the physical capacity you need to do it without the assist.